1. Field of the Invention
The present invention relates to a method and apparatus for correcting the moire effect on a display monitor based on displayed image resolution.
2. Background Information
Color cathode ray tubes (CRTs) are commonly used as visual display devices, employing up to three electrodes, typically one for each primary color: red, green, and blue. Most color CRTs use a shadow mask to selectively illuminate a matrix of each electrode's respective colored phosphors (i.e., red, green, and blue). The beam is typically larger than the shadow mask opening size, so the shadow mask blocks part of the beam and casts a portion of the original beam onto the desired phosphor. The dot pitch, or spacing, between adjacent shadow mask openings is typically between 0.2 and 0.3 mm to achieve a high resolution display.
A video pattern displayed on the CRT is comprised of a array of pixels, typically 640 to 1280 pixels wide and 480 to 1024 pixels high. One pixel will typically illuminate several screen dots. The number of screen dots illuminated is inversely proportional to the resolution of the video pattern. At higher video resolutions the number of screen dots illuminated can be less than three. At these higher resolutions an interference pattern, typically termed the moire effect, can appear. From an operating standpoint, the moire interference phenomenon poses a serious aesthetic problem, since the best electron beam focus and the highest image resolution results in generating an unacceptably noticeable moire effect if the video signal being displayed includes alternating pixel patterns, which is a common occurrence.
The moire effect becomes stronger as the horizontal resolution of the displayed image increases and as the pixel size approaches the size of the screen dot pitch. Accordingly, a larger correction signal is required at higher resolutions. An apparatus that provides a range of adjustment that is large enough for a high resolution image will be overly sensitive when correcting the moire effect of a lower resolution image. Conversely, an apparatus that provides a range of adjustment that is appropriate for a low resolution image will not provide sufficient adjustment when correcting the moire effect of a higher resolution image. Accordingly, what is needed is a moire correction apparatus that provides appropriate moire correction signals for higher resolution and lower resolution images.